This invention relates to magnetic tape cassettes, and concerns in particular a magnetic tape cassette having improved separation sheets for facilitating the smooth movement of the magnetic tape from one spool to the other during operation.
As is well known, a cassette consists of a generally rectangular case made of two equally sized halves, upper and lower, and a pair of tape spools housed in the case, with the tape wound on each spool to take a form of tape reel. The case is closed on all sides but one, and each spool is rotatably mounted on a hub supported by the case wall. In operation, the tape is transported from one spool to the other, the direction of the tape being dependent upon the mode of operation, which may be that of recording, reproducing or rewinding of the device in which the cassette is used.
The tape spool itself typically has no flanges extending alongside the reel of tape on both sides for guiding the running tape, that tape being taken up on or released from the reel by restraining the tendency of the reel-forming tape to displace itself sidewise. Because of the absence of such flanges, it has long been noted that, in this basic cassette construction, interference is likely to occur between the case walls and the tape in motion into or from the reel on each spool, resulting in impairment of smooth tape movement necessary for satisfactory recording and playback operations.
It has been general practice to avoid such interferences and thus ensure the smooth running of the tape by providing a separation sheet between case wall and tape reel; and for the material of the separation sheets, a synthetic material such as polyethylene terephthalate, carbon-impregnated fluorocarbon, carbon-coated polyester, silicone coated paper or any other similar material in sheet film or paper form, with or without a lubricating agent applied thereto, is used.
In order to interpose separation sheets, a clearance must be secured in the cassette design to accommodate the thickness of the separation sheet between each half-case and tape reel. If this clearance is too small, a contact will result between the sheet and the tape or the case wall to introduce friction adverse, in either instance, to the smooth running of the tape. On the other hand, if the clearance is too large, the separation sheets will not serve the purpose of guiding the tape and will allow the tape to be wound to form a disorderly reel or to be unwound and run in a wobbly manner, causing the tape edges to hit the separation sheets and suffer damage. Various improvements have been proposed with respect to the design of the separation sheet with a view to allowing the clearance to be as small as possible and yet minimizing the resultant friction.
In one example, G. D. Abitboul proposed, in U.S. Pat. No. 3,556,433, a bowed or flexed separation sheet. In another, P. G. Schmidt proposed, in U.S. Pat. No. 3,675,875, a sheet having horizontal corrugations. The flexure and horizontal corrugations purport to reduce the friction between running tape and separation sheet in the guiding action performed by the sheets. However, such separation sheets present manufacturing problems in that the flexure and the corrugations are hard to determine for the best friction reducing effect; also, it is difficult to keep the clearance within a practical tolerance in cassettes being mass-produced.
A purpose of this invention is to provide a magnetic tape cassette in which the tape is enabled to run smoothly. Another purpose is to eliminate the problems associated with the static friction of the tape. Still another purpose is to provide a means of ensuring the stable and accurate running motion of the tape. Other purposes are to provide a magnetic tape cassette in which the clearance between tape reel and half-case can be set precisely and to provide a separation sheet easy to manufacture and offering minimized friction in contact with the tape.
In the present invention, a separation sheet having at least one and preferably a plurality of dome-like protrusions spaced throughout the sheet is employed. The protrusions are preferably between about 0.1 and 0.5 mm in height (H) and between about 5 and 10 mm in diameter (R), with an R/H ratio of preferably between about 100 and 10. The separation sheet may advantageously be formed with an additional protrusion on the side thereof opposite from the dome-like protrusions and shaped to fit into a window included in the adjacent inner wall of the cassette case.